7
CLIMATE GROUND COVER
Cut-off lows influence
Victoria's rainfall
CSIRO HAS ESTABLISHED ONE OF
THE KEY FACTORS THAT CONTROLS
RAINFALL IN VICTORIA
BY PETER MCINTOSH
ACCURATELY FORECASTING THE break in the
season, and consequently when to sow, is crucial
for successful grain production. This was clearly
demonstrated in 2006 where crops sown closer to
the break performed substantially better than those
sown later.
Researchers are working with growers from Birchip
in Victoria to understand their regional forecast needs
and to develop a new forecast system that better meets
those needs.
To forecast the break, it is important to understand
the physical processes that transport the moisture and
cause it to be dropped as rain. Analysing 35 years
of data for Birchip, we made a startling discovery
– 51 per cent of north-west Victoria’s growing-season
rainfall is caused by cut-off lows, as opposed to cold
fronts and other weather systems. As much as 70 per
cent of the heavier rainfall events are attributable to
cut-off lows.
A cut-off low is an isolated low-pressure system
that has broken away from the low-pressure belt to
its south, and that extends vertically through much
of the atmosphere. Seasonal rainfall variability can
be the result of a varying number of cut-off lows, a
varying amount of rainfall from each cut-off low, or a
combination of both.
The decrease in north-west Victoria’s rainfall since
about 1995 can be explained by a decrease in cut-off-
low rainfall. It was found that the number of cut-off
lows is influenced by sea surface temperatures to the
south of Australia, while the amount of rain delivered
by each cut-off low is influenced by the sea surface
temperatures to the north.
In spite of the chaotic nature of our climate and
the amount of research that remains to be done, the
research team are optimistic about creating a valuable
forecast system for farmers that overcomes the
limitations of currently available seasonal forecasting
systems (Table 1).
Australia is at the forefront of research into
applying climate science to agriculture. There is now
a methodology for isolating the synoptic weather
systems and a better understanding of the kind of
forecasts that farmers value.
GRDC Research Code LWR25
More information: Dr Peter McIntosh, 03 6232 5390,
peter.mcintosh@csiro.au
TABLE 1: LIMITATIONS OF AUSTRALIA'S CURRENTLY AVAILABLE
SEASONAL CLIMATE FORECAST SYSTEMS
Seasonal climate forecast system
Limitation
Bureau of Meteorology's monthly seasonal
outlook based on sea surface temperatures
in the Pacific and Indian Oceans.
On-farm accuracy, averaged over many
rainfall stations across Australia, has
recently been questioned by some
scientists.
Forecasts based on the Southern
Oscillation Index.
Are limited to a three-month lead-time.
The Southern Oscillation Index phase
system.
Is more accurate in north-east Australia
where ENSO has greatest impact.
The ENSO (El Niño--Southern Oscillation)
sequence system.
Shows promise but, being ENSO-related,
applicability may be limited to north-east
Australia.
The Bureau of Meteorology's Predictive
Ocean Atmosphere Model for Australia
(POAMA).
Predicts Pacific Ocean sea surface
temperatures up to 12 months in advance.
Farmers have to know how to translate this
into something relevant to their decision-
making.
Some forecasts can be most accurate at a time when farmers cannot react (they may
have already sown). The reverse can also be true; for example, the March--May ENSO-
based forecast has no accuracy beyond that time, which is when farmers most need it
(for sowing); it can only be used for top-dressing decisions later in the year.
FORECASTING
A cut-off low is an isolated low-
pressure system that has broken
away from the low-pressure belt
to its south, and extends vertically
through much of the atmosphere.
Seasonal rainfall variability can be
the result of a varying number of
cut-off lows, a varying amount of
rainfall from each cut-off low, or a
combination of both
EMMA LEONARD